In the fabrication of thin film wiring for electronic applications, it is often required to etch polycrystalline copper surfaces. Any roughening of this surface can cause significant process or electrical variability to the structures. Current etchants show significant surface roughening, as well as dependence of etching rates on feature density, size, electrical connectivity, and location on the wafer. In particular, commonly used copper etchants, such as acid solutions of peroxide or persulfate, tend to preferentially etch the copper in the immediate proximity of grain boundaries. In addition, different exposed crystal faces tend to etch at different rates.
Numerous applications in semiconductor fabrication would benefit from being able to provide smooth, sub-micron recess etching of copper and copper alloys. Some applications in semiconductor fabrication that would benefit for such a process include:                (1) Selective capping in advanced CMOS devices: to decrease the effective dielectric constant of the intra-layer insulator, and also to increase the strength of the via-to-line junctions.        (2) Post-CMP (chemical mechanical polishing) clean for copper Dual Damascene builds: to suppress dendrite growth for increased reliability, and to remove copper residues on dielectric to decrease current leakage and increase shorts yield. Although commercial cleaning solutions exist for this, these are usually extremely costly.        (3) Anchored vias (inverted mushroom): to increase mechanical strength of via structure.        (4) Post-CMP cleanup for FBEOL selective (Ni, Au) plating on copper-maskless technology, whereby the processing does not roughen the copper seed layer in the features where the subsequent plating will be done, while allowing the complete removal of copper left from the CMP process from the top surfaces.        (5) Post-CMP cleanup for deep via selective Cu plating—system on a chip.        
Accordingly, a great need exists for providing a process for isotropic, smooth sub-micron etching of copper and copper alloys.